Earth's Largest Extinction Events

The Permian-Triassic extinction, which took place 251.4 million years ago, was the Earth's largest, with 96% of all marine species and 70% of land species dying off. It's also the only known mass extinction of insects. Possible causes:

Methane Hydrate Gasification: the ocean sea-floor contains large amounts of methane hydrates, if those were to be released by a temperature change, the global temperature would increase by 6 degree Celsius (10.8F).

Flood Basalt Eruptions: Siberian Traps was one of the largest volcanic eruptions in Earth's history, with over 2 million square kilometers covered in lava.

Impact Event: Major asteroid collision with Earth has been proposed as a possible cause but no large impact crater has been located within the right time-period; however, if the asteroid impacted an ocean, the crater would have been destroyed by "conveyor belt" sea-floor spreading. The oldest sea-floor is only 200 million years old.

The Ordovician Silurian Extinction was actually two extinction in close proximity that took place 440-450 million years ago, killing off over 60% of all marine species. Possible causes:

Weathering: heavy weathering of the Appalachian Mountains out-weighed volcanism, resulting in abnormally large CO2 sequestering, which lead to the development of a rapid ice age.

Gamma Ray Burst: this is a speculative hypothesis, but a gamma ray burst from a hypernova within 6,000 light years could have irradiated Earth. Within 10 seconds of the burst, half of the ozone layer would have been destroyed, bathing the planet in ultraviolet radiation.

The Cretaceous Tertiary event, taking place 70-65 million years ago, killed off 75% of all species, including the dinosaurs. This extinction cleared the way for mammals and birds, where before dinosaur were the dominant land species. Avian dinosaur fossils are the only ones found after the K-T boundary. Possible causes:

Impact Event: an asteroid 10-15 km or 6-9 miles in diameter impacted Earth at Chicxulub, in Mexico's Yucatan Peninsula. The explosion would have been a billion times larger than the nuclear bomb dropped on Hiroshima.

Flood Basalt Eruption: Deccan Traps flood basalt erupted for 800,000 years during the K-T boundary period, it would have exacerbated the devastation caused by the asteroid impact and would have slowed down the recovery of organisms.

A series of extinctions 360-375 million years ago, removed 70% of all species. Major environmental changes including frequent sea level changes, anoxic ocean bottoms, abnormally large carbon burials all contributed to the extinction; however it is not clear what caused the environmental changes in the first place. Possible causes:

Plant evolution - Devonian sough rapid evolution of plants, in the early Devonian the tallest plants were 30 cm but by late Devonian plants escalated to 30 m (90 feet). Archaeopteris forests quickly covered the globe.

Effect on weathering - these plants contributed to heavy weathering as plant roots broke up soil/rock releasing huge quantities of nutrients in to the rivers and the ocean, the ecology of the time was not able to absorb the nutrients subsequently leading to anoxia.

Effect on CO2 - the greening of the planet absorbed CO2 leading to a permanent chilling to the planet.

About 199.6 million years ago 55% of all species went extinct, this event marks the shift from Triassic and Jurassic periods. Possible causes:

Impact Event: to date no impact crater has been found, the Manicouagan Reservoir in Canada, was an impact 12 million years before the extinction so is not the cause.

Flood Basalt Eruption: Central Atlantic Magmatic Province eruption is considered the most likely cause of the extinction, also as in the Permian-Triassic Extinction the gas hydrates might have played a roll.

Between 600 to 750 million years ago Earth went though up to 4 complete glaciation periods, each time ending with extreme green house conditions, this late Precambrian planet-wide glaciation is known as "Snowball Earth."